The present invention relates to ion implanters and, more particularly, to an ion implanter for separation by implanted oxygen (SiMOX) which is suitable for implantation of oxygen ions into a silicon wafer.
There has been conventionally known an ion impanter which implantes oxygen ions into a silicon wafer to form an insulating film of silicon dioxide within the silicon wafer. In this type of ion implanter, however, when oxygen ions are implanted into the silicon wafer, this causes the silicon wafer to be charged positively so that discharging takes place between the silicon wafer and a wafer holder, thus forming a discharge mark on the rear side of the silicon wafer. To avoid this, there has been employed an ion implanter of such an arrangement that, upon directing an ion beam onto a silicon wafer to implant ions into the wafer, an electron beam is also irradiated onto the silicon wafer to neutralize the silicon wafer charged by the ion beam with use of the electron beam, as disclosed in JP-A-3-194841 or JP-A-8-96744.
It is therefore an object of the present invention to provide an ion implanter which can direct only an electron beam toward a silicon wafer by separating impurity ions from electrons generated from an electron generation source.
In the prior art, since the electron beam is directed together with the ion beam toward the silicon wafer in order to neutralize the charged silicon wafer, it can be prevented that charging cause generation of particles. In the prior art, however, no consideration is paid to trapping impurities generated from a filament as an electron generation source. For this reason, impurity ions, e.g., tungsten ions, generated together with thermions from the filament are implanted together with electrons into the silicon wafer, resulting in that the silicon wafer is polluted with the metal. The metal pollution of the silicon wafer causes degradation of insulating characteristics of the wafer and therefore an involved insulation fault leads to reduction of the quality of the wafer.
The present invention solves such a problem in the prior art.
In accordance with an aspect of the present invention, the above object is attained by providing an ion implanter which comprises an ion beam generation means for directing an ion beam from an ion generation source toward a wafer, an electron beam generation means for converting electrons generated from an electron generation source into an electron beam and outputting the beam, and an electron beam irradiation means for separating the electron beam from an impurity ion beam generated from the electron beam generation means as associated with the electron beam to irradiate only the electron beam onto the wafer to cause ions in the ion beam to be implanted into the wafer.
The electron beam irradiation means in the ion implanter may be arranged to have a function of separating the electron beam generated from the electron beam generation means and the impurity ion beam generated as associated with the electron beam to trap impurity ions in the impurity ion beam and irradiating the electron beam onto the wafer, or to have a function of separating the path of the electron beam generated from the electron beam generation means and the path of the impurity ion beam generated as associated with the electron beam to trap impurity ions in the impurity ion beam and irradiating the electron beam onto the wafer.
The ion implanter may additionally include elements (1) to (11) which follow as necessary.
(1) The electron beam irradiation means applies a magnetic field to the electron beam and impurity ion beam to deflect the both beams based on their masses.
(2) The electron beam irradiation means includes a magnetic circuit which is disposed in the middle of a beam transmission path connecting said electron beam generation means and said wafer for variably changing a magnitude of the electron beam in the beam transmission path.
(3) The magnetic circuit includes a magnet for generating a magnetic field and a pair of cores disposed as opposed each other on both sides of the magnet for establishing a magnetic field from the magnet in a direction intersected with the beam transmission path, and the pair of cores are shaped into a triangle which area becomes small as it goes away from the magnet.
(4) The electron beam irradiation means applies an electrical field to the electron beam and impurity ion beam to deflect the both beams based on their masses.
(5) The electron beam irradiation means includes positive and negative electrode plates which are disposed in the middle of the beam transmission path connecting the electron beam generation means and said wafer and form a curve in the beam transmission path.
(6) A lens system for adjusting a diameter of the electron beam is provided in the beam transmission path on a side of the positive and negative electrode plates close to the wafer.
(7) The electron beam irradiation means includes a trap plate which traps impurity ions at a location intersected with the transmission path of the impurity ion beam.
(8) An anti-adhesion plate made of the same material as the wafer is fixedly mounted on the trap plate.
(9) The ion beam generation means and said electron beam irradiation means carry out irradiation and stoppage of the ion beam and electron beam synchronously respectively.
(10) The electron beam irradiation means includes a temperature detection means for detecting a temperature of the electron beam to be irradiated onto the wafer and a magnetic field adjustment means for controlling the magnitude of the magnetic field according to a detection output of the temperature detection means.
(11) The electron beam irradiation means includes a temperature detection means for detecting a temperature of the electron beam to be irradiated onto the wafer and an electrical field adjustment means for controlling the magnitude of the electrical field according to a detection output of the temperature detection means.
(12) The electron beam irradiation means includes a temperature detection means for detecting a temperature of the electron beam to be irradiated on to the wafer and a magnetic field adjustment means for controlling the magnitude of the magnetic field according to a detection output of the temperature detection means.
In the above means, before the ion beam generated from the ion generation source is irradiated onto the wafer, electrons emitted from the electron generation source are converted to the electron beam, electrons emitted from the electron generation source are separated from impurity ions emitted therefrom as associated with generation of the electrons, and only the electron beam is irradiated onto the wafer. As a result, it can be avoided that the wafer be polluted with the impurity metals and thus the insulating characteristics of the wafer be degraded, thus enabling contribution to an improvement in the quality of the wafer. Further, in order to separate the electron beam from the impurity ion beam, the separated impurity ions are trapped to irradiate only the electron beam onto the wafer, or the path of the electron beam is separated from the path of the impurity ion beam to trap the separated impurity ions to irradiate only the electron beam onto the wafer. Thus the wafer can be neutralized without being polluted with the impurity metals, contributing to an improvement in the quality of the wafer.
As has been explained above, in accordance with the present invention, when an electron beam is directed together with an ion beam toward a wafer, electrons are separated from impurity ions generated as associated with the electrons to direct only the electron beam toward the wafer. Therefore, the charging of the wafer can be neutralized without pollution of the wafer with the impurity metals, which contributes to an improvement in the quality of the wafer.